1. Field of the Invention
The present invention relates to a piston ring for an internal combustion engine.
2. Description of the Related Art
A carbon-based coating is known as a coating which has a low coefficient of friction, but the coating stress is high and self fracture occurs, so increasing the thickness is difficult. Japanese Patent Publication (A) No. 2006-008853 proposes a carbon-based coating with a low coefficient of friction which is made a multilayer structure with a surface-most layer comprised of a soft layer and with soft layers and hard layers alternately laminated and which enables a smooth surface to be easily obtained.
See the above Japanese Patent Publication (A) No. 2006-008853 for the related art.
Japanese Patent Publication (A) No. 2006-008853 does not define the hardness difference between the hard layer and the soft layer. However, in Japanese Patent Publication (A) No. 2006-008853, the soft layer is meant as a fitting layer, while the hard layer is meant as a wear resistance layer, so the hardnesses are, in Example 1, soft layer: 18 GPa and hard layer: 42 GPa and, in Example 2, soft layer: 20 GPa and hard layer: 41 GPa. The hardness difference between the soft layer and hard layer was large.